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Cofactor (biochemistry)
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A cofactor is a non-protein chemical compound that is bound (either tightly or loosely) to an enzyme and is required for catalysis. They can be considered "helper molecules/ions" that assist in biochemical transformations. Certain substances such as water and various abundant ions may be bound tightly by enzymes, but are not considered to be cofactors since they are ubiquitous and rarely limiting. Some sources limit the use of the term "cofactor" to inorganic substances.
Cofactors can be divided into two broad groups: organic cofactors coenzymes and metal ions (usually Mg2+, Cu+, Mn2+ for instance).
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A cofactor is a non-protein chemical compound that is bound (either tightly or loosely) to an enzyme and is required for catalysis. They can be considered "helper molecules/ions" that assist in biochemical transformations. Certain substances such as water and various abundant ions may be bound tightly by enzymes, but are not considered to be cofactors since they are ubiquitous and rarely limiting. Some sources limit the use of the term "cofactor" to inorganic substances.
Cofactors can be divided into two broad groups: organic cofactors coenzymes and metal ions (usually Mg2+, Cu+, Mn2+ for instance). Coenzymes are small organic non-protein molecules that carry chemical groups between enzymes. Cofactors (both coenzymes and metal ions) can be either loosely or tightly (often covalently) bound to the enzyme. In the latter case they can be called prosthetic groups.
Note that the terms cofactor, coenzyme and prosthetic group represent three different, though sometimes overlapping concepts. The terms cofactor (the more general term) and coenzyme (organic molecule) refer to enzymes and as such to the functional properties of a protein. On the other hand "prosthetic group", emphasizes the nature of the binding (tight or covalent) and thus refers to a structural property. For instance, you would not call the sugar moiety of a glycoprotein, a cofactor, as it has no enzymatic activity. On the other hand, the covalently bound coenzyme FAD is a prosthetic group, while the generally loosely bound coenzyme NAD+ is a cosubstrate but not a prosthetic group.
Apoenzymes and holoenzymes
The inactive protein, without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is the holoenzyme.
Apoenzyme + cofactor <=> Holoenzyme
Metal ion cofactors
Metal ions are common cofactors. The study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list commonly includes iron, manganese, cobalt, copper, zinc, selenium, and molybdenum. Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified. Iodine is also an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor. Calcium is another special case, in that it is required as a component of the human diet, and it is needed for the full activity of many enzymes: such as nitric oxide synthase, protein phosphatases or adenylate kinase, but calcium activates these enzymes in allosteric regulation, often binding to these enzymes in a complex with calmodulin. Calcium is therefore a cell signaling molecule, and not usually considered as a cofactor of the enzymes it regulates.
Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter, tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus, and even cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii.
In many cases, the cofactor includes both an inorganic and organic component. One diverse set of examples are the haem proteins, which consists of a porphyrin ring coordinated to iron.
Coenzymes
A coenzyme is a small organic molecule (typically a molecular mass less than 1000 Da) that can be either loosely or tightly bound to the enzyme and directly participate in the reaction. In the latter case, when it is difficult to remove without denaturing the enzyme, it can be called a prosthetic group. It is important to emphasize that there is no sharp division between loosely and tightly bound coenzymes. Indeed, a typical coenzyme like NAD+ can be tightly bound in some enzyme, while it is loosely bound in others. Thiamine diphosphate (ThDP) is tightly bound in transketolase or pyruvate decarboxylase, while it is less tightly bound in pyruvate dehydrogenase. Other coenzymes, flavin adenine dinucleotide (FAD), biotin or lipoamide for instance, are covalently bound. Tightly bound coenzymes are generally regenerated during the same reaction cycle, while loosely bound coenzymes can be regenerated in a subsequent reaction catalyzed by a different enzyme. In the latter case, the coenzyme can also be considered a substrate or cosubstrate. Vitamins can serve as precursors to coenzymes (e.g. vitamins B1, B2, B6, B12, niacin, folic acid) or as coenzymes themselves (e.g. vitamin C).
Non-enzymatic cofactors
The term is used in other areas of biology to refer more broadly to non-protein (or even protein) molecules that either activate, inhibit or are required for the protein to function. For example, ligands such as hormones that bind to and activate receptor proteins are termed cofactors or coactivators, while molecules that inhibit receptor proteins are termed corepressors.
See also
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